The development of multi-drug resistance (MDR) is a major cause of failure in chemotherapeutic management of cancer. In breast cancer, for instance, more than 50% of the patients relapse due to acquired resistance to standard chemotherapy regimens. Novel strategies to overcome MDR in a clinically meaningful way that does not expose the patients to significant toxicity are urgently needed. In the proposed study, our strategy to overcome MDR in vivo relies on a multifunctional approach to optimize delivery of pro-apoptotic drugs to the tumor mass, increase the intracellular drug concentrations, and reverse cellular resistance by modulating ceramide levels. The preliminary studies show that we can prepare tumor-targeted biodegradable polymer-based engineered nanocarriers (PENS) for encapsulation of hydrophobic pro-apoptotic drugs, like paclitaxel. We have also found that increasing intracellular ceramide concentrations by delivery from exogenous source or inhibiting the metabolism results in significant enhancement of cytotoxicity in sensitive and resistant tumor cells. C6-ceramide and tamoxifen, a potent inhibitor of ceramide metabolism, were co-administered with paclitaxel for synergistic activity in tumor cells and in vivo. Based on these preliminary findings, we are confident that PENS, developed using engineering design criteria, can be made to efficiently deliver multiple therapeutic agents to the tumor mass. The specific aims of the proposal are: (1) to develop, characterize, and optimize long-circulating, biodegradable polymeric nanocarriers with encapsulated paclitaxel, ceramide, and tamoxifen, either alone or in combination, (2) to evaluate the uptake, distribution, intracellular concentrations of paclitaxel, ceramide, and tamoxifen, cytotoxicity, and apoptotic activity in culture of sensitive and resistant tumor cells, (3) to examine the biodistribution and pharmacokinetic profiles of drugs administered in the control and nanocarrier formulations in sensitive and resistant xenograft tumor models established in nude mice, (4) to determine the antitumor efficacy of single and combination therapy in PENS in sensitive and resistant xenograft models, and (5) use mathematical modeling to improve the design of nanocarriers for tumor-targeted delivery of single and combination drug therapy. The results of this study would be extremely valuable in the treatment of refractory tumors using a multifunctional nanotherapeutic approach that efficiently delivers the drug and can overcome cellular resistance. The multimodal nanocarrier strategy proposed here would provide a translatable approach to overcome MDR in cancer patients.